Data collected in the NCPP have been analysed to define the primary and secondary contributions of specific biological and environmental variables to mental retardation in a population of 37,000 children followed from the prenatal period to age 7. The frequency of severe retardation was not found to differ by ethnic group, but mild retardation was more frequent among blacks than among whites. The incidence of mental retardation was negatively related to social class. Major neurological abnormalities were more frequent among the severely retarded than midly retarded children, and more frequent among whites than blacks in both of the retarded groups. Within ethnic group, the proportion of neurologically involved retarded children was found to increase with social class. Identified risk factors for mental retardation include urinary tract infections during pregnancy, teen-age pregnancy, clinical signs of perinatal anoxia, and poor psychomotor performance in infancy.